This invention generally relates to the concentration in time of a gas component entrained in a carrier gas with which it forms a mixture and, in particular, to method of and apparatus for effecting such concentration by flowing the mixture through an electrically conductive hollow body of low thermal capacity which is cooled by a solid phase device during a condensation time interval and is heated during a desorption time interval by an AC current passing therethrough, the latter time interval being very short compared with the former.
Concentration in time as practised in the art varies in practical details, depending on the requirements of a specific application. In general, the mixture is passed through a cooled hollow body or "cold trap" containing a suitable adsorbent in order to condense the gas component molecules out of the carrier and onto the adsorbent for a suitably long time. Then the temperature is quickly raised and held for a short time; as a result, the molecules are thermally desorbed at a high rate and reentrained in the carrier gas. In this way the number of component gas molecules per unit time conveyed by the carrier emerging from the cold trap due to desorption is increased compared with the molecules per unit time conveyed by the carrier when entering the trap. If, for example, the application is the testing for impurities of gas chromatographic carrier gas, the trap may be used in a continuous mode, in which the gas is flowed at a steady rate straight from a supply cylinder through the trap and some suitable analytical instrument, such as a gas chromatograph, during both condensation and desorption times, after it has been established experimentally that the condensation time is significantly less than would be required for exceeding the condensation capacity of the trap. In other applications, the trap may be associated with gas switching circuits to interrupt gas flow between condensation and desorption or perform other flow control functions.
Although concentration in time often requires cooling the trap below ambient temperature, this is not always the case. If the gas mixture needs to be heated well above ambient temperature in order to keep a given gas component entrained in the carrier gas, condensation of the component will take place if the trap temperature is sufficiently depressed with respect to the temperature of the mixture. It is easy to appreciate, therefore, why a trap as hereinbefore referred to is usually called a cold trap, i.e. cold with respect to the temperature of the gas admitted for condensation, even though it may in fact be held at above ambient temperature.
It is believed that the phrase "concentration in time" aptly suggests trapping molecules of interest over a comparatively long time interval, say, five minutes, and releasing them in a few seconds. In other words, the molecules that could be counted in the mixture flowing in during the five minutes of the condensation stage could also be counted in the mixture flowing out during the few seconds of the thermal desorption stage. It could, of course, be argued that the phrase "concentration in volume" is equally applicable, in that the molecules that are found in the comparatively large volume of mixture that flows in during condensation are also found in the much smaller volume that flows out during desorption, the disparity in volume being of course the result of the disparity in duration between the two stages. If we assume that the flow of the mixture is not only continuous but also constant, any Gas Chromatograph supplied via the trap will naturally receive a plug of mixture in which the molecules of interest are in higher volumetric concentration than would be the case for the same flow if the trap were not interposed.
Any hollow body of low thermal mass and convenient geometry, i.e. a longitudinally extending hollow body, may be used as a cold trap in the concentration method and apparatus referred to, provided it can withstand the desorption temperature and is substantially inert to the mixture that is to flow therethrough. In order to enhance its efficiency, the hollow body may be packed in part with a material that is a good adsorbent for the gas component to be condensed but not for the carrier. A known trap comprises a hollow body in the form of a thin-walled stainless steel U-tube packed with an adsorbent such as Tenax (R.T.M.) in the bend region, the limb space being left free. One end of the tube represents the inlet for the unconcentrated mixture and the other the outlet for the concentrated mixture. Both ends are provided with connecting means for inserting the trap in a suitable gas circuit so that, for example, a sample gas component in a carrier may be routed to some analytical instrument, such as a gas chromatograph, after the component has been concentrated in time, with the result that a chromatogram benefiting from better signal-to-noise ratio may be obtained.
In a known prior art arrangement making use of a U-tube as referred to, the tube may be cooled to a low temperature either by immersing it in a Dewar containing liquid nitrogen or by offering the Dewar to the tube. The gas condensation stage is extended over a period of minutes, at the end of which the tube is heated up for something less than half a minute, to effect thermal desorption of the condensed gas and thus complete the concentration process. In this case, no dwell is interposed between condensation and desorption.
It is clearly important to make the transition from the temperature of the condensation stage to the much higher temperature of the desorption stage as rapidly as the nature of the condensed gas permits since the effectiveness of the concentration process depends on it. In the prior art referred to, desorption is effected by ohmic heating of the bend region, which to this end is permanently connected through heavy leads to the secondary of a large step-down transformer, the primary being fed from the public AC supply for the duration of the desorption stage.
There are several disadvantages connected with the prior art method and apparatus. Firstly, the use of liquid nitrogen for cooling is unacceptable for routine applications in an industrial environment: the Dewar needs to be replenished from time to time and the concentration apparatus, which could well form part of a pollution analysing system, cannot therefore be left unattended overnight unless expensive automated means for topping up the Dewar are employed. Secondly, the displacement of the Dewar, or worse still the U-tube, is an inconvenient mechanical operation giving an ill-defined transition between cooling and heating. Thirdly, the use of ohmic heating at the frequency of the public supply means in effect that the secondary of the bulky step-down transformer thus required cannot be brought close enough to the U-tube to allow the use of very short leads. The comparatively long leads in turn compel the use of a very heavy gauge in order to avoid excessive voltage drop. Such massive leads, in a good heat conductor such as copper, impose a considerable thermal loading on the bend region of the U-tube and cause undesirable heat gradients at the points of attachment to the tube, leading to recondensation of the desorbed gas and, therefore, lowering of the concentration efficiency. The high thermal loading is also objectionable because it is wasteful of cooling effort, i.e. cooling of the bend region takes longer or for a given cooling time the temperature cannot be depressed quite as far as would be the case if the loading were considerably lower. Furthermore, the bulk involved in both the heating and cooling arrangements of the prior art referred to is a serious drawback which makes it impracticable to incorporate them in compact apparatus such as analytical instruments.